1. Field of the Invention
The present invention relates to a method and a control device for operating an internal combustion engine.
2. Description of Related Art
In the article “Die neue Emissionsstrategie der Benzin-Direkteinspritzung” (“The new emissions strategy of direct gasoline injection”), MTZ 11/2003, pp. 916-923, the possibility of a “stratified start” is described. In contrast to a homogenous low-pressure start, in which injection takes place during an intake stroke, in a stratified start the injection does not take place until late during the compression stroke. This results in a stratified fuel-air mixture having a rich mixture cloud in the area of the spark plug.
The advantage of such a stratified start in comparison with a conventional low-pressure start is that, above all at low ambient and engine temperatures, a lower enrichment is required, which improves the emission characteristic of the internal combustion engine. Such an enrichment is required because during startup a portion of the injected fuel adheres to the cold combustion chamber walls as a film, and thus does not fully take part in the actual combustion during the start phase. The fuel that remains uncombusted in this way is ejected without being combusted and results in undesirable hydrocarbon emissions. In order to compensate the fuel mass that does not participate in the combustion, the injected quantity must be correspondingly increased. Given a homogenous low-pressure start, enrichment factors of 2 to 3 over the stoichiometric quantity are not unusual.
After the stratified start phase, which usually lasts approximately 1 to 2 seconds, i.e. after the very first injections and ignitions, a catalytic converter heating phase takes place during which a homogenous split injection is carried out. In the above-named article, concerning this it is proposed that a first injection take place during the intake stroke, producing a lean homogenous base mixture in the combustion chamber. There then follows a second injection during the subsequent compression stroke, which provides a rich mixture cloud in the area of the spark plug. The ignition takes place relatively late, shortly after top dead center, between the compression stroke and the following expansion stroke. In this way, a late center of the combustion is realized, with the result that a large part of the released energy is not converted into mechanical energy, but rather is released in the exhaust gas as heat. This enables a very rapid heating of a catalytic converter. The production of the charge stratification is in both cases preferably produced by a spray-guided method.
From published German patent document DE 10 2004 017 989, it is known, in operation with a very lean charge mixture, to make a pre-injection during an intake stroke and to make a main injection during a compression stroke immediately before ignition. However, here the mixture preparation is not optimal.